Devices designed for off-normal, directed optical characteristics such as transmission, reflectance, contrast, brightness etc. are well known in the prior art. The optical anisotropy of nematic liquid crystals makes them ideal for use in these devices. The high level of development of liquid crystal materials and application methodology provide a solid technological foundation and facilitate their use in directed, light-modulating optical devices.
In liquid crystal displays, light from front- or backlighting system or from ambient light sources sequentially passes through a plurality of functional layers. In the case of transmissive LCDs, the functionality of the display requires at least a polarizer, a liquid crystal (LC) layer confined between transparent electrodes, and another polarizer. Reflective LCDs may be constructed such that either one or two integral polarizers are required. Other required elements include alignment layers that provides orientation of directors of liquid crystal molecules on the boundaries of the liquid crystal and transparent substrates to protect the liquid crystal and other layers confined between them from mechanical and other damages. Reflective LCDs include an additional reflective layer. Additionally, a plurality of functional layers such as retardation plates, color filters, planarization and protective layers, insulating layers and other layers may also be included in the display depending on the technical requirements to the display and its functions.
Many types of liquid crystal configurations are known which are capable of modulating the intensity of light and providing controlled contrast for the display of images. The most common and generally most effective LC devices for this function are based on liquid crystals in the nematic phase, operating with the use of the twist effect, that is, a twist of the polarization plane of the light passing through a layer of liquid crystal material. The principle of operation of a twisted-nematic liquid crystal cell is based on the use of a pair of polarizers employed together with a voltage-driven LC cell. A front polarizer polarizes the incident light. The polarization plane of the polarized light is twisted through a certain angle as polarized light passes through the liquid crystal layer before it meets a second polarizer. The second polarizer is also called the analyzer. A change in the voltage across the liquid crystal results in a change in a twist angle of the polarization plane, as light passes through the liquid crystal layer. This allows the light intensity to be controlled at the exit from the display by changing an angle between the light polarization plane at the exit from the liquid crystal and the transmission axis of the analyzer.
The capability to modulate the intensity of the light passing through the functional layers of the display is realized with the liquid crystal confined between transparent electrodes, which are in turn confined between the transparent substrates and the entrance and exit polarizers. Depending upon the particular LCD application, the entrance and exit polarizer may be oriented with their transmission axes crossed, in which case the operating mode is called a normally-white (NW) mode, or the two polarizers may be oriented with their transmission axes aligned parallel to one another, in which case the operating mode is designated as normally-black (NB) mode. In the case of a twisted-nematic (TN) LCD operated in the NW mode, if a voltage applied to the LC layer with the aid of electrodes completely suppresses the twist effect, the polarization of light created by the first polarizer remains unchanged and the light is absorbed in the second polarizer oriented perpendicularly to the first one (crossed polarizers). On the contrary, when no voltage is applied to the LC, the polarization plane of the light is rotated so that the beam passes through the second polarizer without absorption. In the case of a TN LCD operated in the NB mode, the relations between the applied voltage and light throughput are the reverse of those described for the NW mode.
The above scheme can exhibit significant variations depending on features of the LCD design. There are two types of LCDs: reflective and transmissive. Reflective LCDs use the light from ambient sources and employ no special backlighting systems, thus consuming minimum energy. Transmissive displays are provided with backlighting systems employing light sources situated on the side opposite to an observer. A reflective display with semitransparent mirror and a backlighting system behind it can operate in both reflection and transmission modes. LCDs of this hybrid type are called transmissive-reflective or transflective LCDs.
In describing LCDs, it is convenient to differentiate between front and rear sides. The front side is that facing the viewer and, in the case of reflective LCDs, the front side also faces the source(s) of ambient illumination. The rear side is opposite to the front side and, in the case of transmissive LCDs, the rear side faces the backlighting system. The set of layers in the LCD structure situated in front of the LC layer is frequently referred to as the front panel, while the layers behind the LC layer are called the rear panel. Accordingly, the like functional layers situated in rear and front panels are specified as “rear” and “front”, such as rear and front substrates, rear and front electrodes, etc.
Many modem liquid crystal displays use the liquid crystal in the so-called mixed mode. The term “mixed mode” designates the mode of the liquid crystal when the Mauguin condition is violated, such that the liquid crystal layer no longer functions as a simple polarization rotator but rather operates as a “mixture” of both a polarization rotator and a birefringent slab or waveplate. In this case, the liquid crystal layer retardation is close to the light wavelength by the order of magnitude:(ne−no)d≈λne and no denote the refractive indexes for extraordinary and ordinary rays in the liquid crystal, respectively, d is the thickness of the liquid crystal, λ is the visible light wavelength (400-700 nm). Here, the retardation of the liquid crystal layer denotes the product of the liquid crystal layer thickness and the difference between extraordinary and ordinary rays refractive indices (i.e., the birefringence of the liquid crystal layer). It should be noted that satisfaction of the Mauguin condition generally requires that the retardation of the liquid crystal layer be much greater than the wavelength of visible light, a condition that is seldom fulfilled given the refractive indices of most liquid crystal materials and the desirability of small liquid crystal cell gaps.
The mixed modes allow development of the liquid crystal light devices with high brightness, high multiplexing, enhanced angular characteristics, better color rendering and other advantageous features. In order to achieve the advantages, the parameters of the liquid crystal display require careful tuning. The physical reason here is the sophisticated light transformation in the liquid crystal. For example, in the general case the linearly polarized light transmitted through the mixed-mode liquid crystal is transformed to elliptical polarization with the additional result that the polarization state at the output of the liquid crystal layer exhibits a strong wavelength dependency. Therefore, a clear, achromatic light output in the optically active state of the liquid crystal display (bright state for the NW mode or dark state for the NB mode) is not possible in the general case.
In order to realize the performance advantages of mixed-mode LCD operation and mitigate the described shortcomings, numerous theories and concepts were developed. Herewith we denote the parameters engaged in the tuning of the liquid crystal mode in order to obtain the best optical and viewing angle performance. Liquid crystal layer parameters engaged are the twist angle, pre-tilt angle, and the retardation determined here as the product of the thickness and the birefringence of the liquid crystal layer. Principal tunable parameters for the polarizers and the alignment layers are the angles between front and rear polarizer transmission axes and, respectively, the rubbing directions of the front and rear alignment layers. Usually the liquid crystal birefringence, the liquid crystal layer thickness, the twist angle and polarizers angles are isolated as the main parameters, although the pre-tilt angle also plays a role in adjusting the optical and viewing angle performance of the LCD operated in a mixed mode.
In addition, the use of different optical phase compensators—i.e., optical retarders—can affect the LCD optical performance. The retarders are used in order to widen the viewing angle of the liquid crystal devices, to obtain the better contrast and throughput luminance, to provide better color rendering, and to enhance other angular characteristics, etc. The different kinds of optical retarders such as stretched-film quarter- and half-wave plates, twist-discotic liquid crystals, Fuji-film and so on are designed to enhance various characteristics of the liquid crystal optical devices.
In the context of the background, all of these enhancements are of two different means. On the one hand, the widening of the contrast viewing angle or viewing cone of any type of liquid crystal device would give a technical solution for some of the particular tasks in the field of the disclosed invention. Here we touch the intersection between the spheres of use of directional light/contrast modulators and wide-angle light/contrast modulators. Although the use of the devices with wide viewing angle is possible in the cases when a specific off-normal viewing direction is required, this is generally not the best solution due to excessive light losses at non-viewable directions as well as unnecessary complexity and cost. On the other hand, in most cases the use of LCDs, which are specially designed and optimized for specific off-normal viewing directions should be substantially less expensive and more practical. The technical art considering the modification of the liquid crystal cell viewing properties by tuning the parameters of the liquid crystal itself and related details could be quite useful for the disclosed invention. The reason is the simplicity of the described solutions.
U.S. Pat. No. 5,280,371 describes a method using a directional diffuser to gather light passing trough a liquid crystal display device in an inclined direction. The method consists of the use of a microlens array in conjunction with a conventional lambertian light diffuser installed sequentially along the light path. The lenses in the array possess parallel optical axes, thus having one common optical axis. This combination is inserted into the plurality of layers of the liquid crystal display in such a way that the microlens array is turned to the light source, and the lambertian diffuser is placed immediately behind the front glass substrate of the liquid crystal. When the common optical axis of the microlens array is turned from the parallel with the light path in the liquid crystal display, the area with maximum concentration of light is turned and aligns in parallel with the optical axis of the microlens array. The main drawback of the device is the technological complexity of the microlens array manufacturing and the associated cost.
U.S. Pat. No. 6,380,995 B1 describes a device, the directional viewing characteristics of which are realized with the aid of a reflective electrode layer disposed on an uneven surface. This electrode has a transparent portion substantially facing in a main viewing angle direction. The main disadvantage of the device is it can only be used in reflective type liquid crystal displays. Transmissive type liquid crystal displays and the other liquid crystal devices without reflective functional layers are not compatible with this approach. In addition, the fabrication of a reflective, partially transparent electrode with an uneven surface structure is technically challenging and expensive.
U.S. Pat. No. 4,890,902 describes a specially designed optical light modulating material comprising an organic resin with microdroplets of liquid crystal material incorporated into a synthetic resin matrix. The index of refraction of the matrix is matched or mismatched to an index of refraction of the liquid crystal optical axis within the microdroplets so that when the microdroplet LC director is aligned relative to a surface of the material, maximum transmission of light occurs at a selected oblique angle relative to the surface of the material or a selected narrow angle about the perpendicular of the material. The use of this material in liquid crystal cells, light control devices for windows and displays provides the devices with an oblique viewing angle, therefore having the potential capability to direct the light to off-normal directions. The drawback of the polymer dispersed liquid crystal or PDLC, is the relatively high operating voltage, low contrast ratio and a highly scattering light characteristic in the voltage off state leading to high levels of diffuse reflection.